where name is a new symbol, each fieldi is a symbol, each
typei is either a type-spec or (ARRAYtype-spec(max)),
each vali is an object satisfying typei, and each bi is t or nil.
Each pair :initially vali and :resizable bi may be omitted; more
on this below. The alist argument is optional and allows the user
to override the default function names introduced by this event.
The doc-string is also optional. The inline-flag Boolean argument
is also optional and declares to ACL2 that the generated access and update
functions for the stobj should be implemented as macros under the hood (which
has the effect of inlining the function calls). We describe further
restrictions on the fieldi, typei, vali, and on
alist below. We recommend that you read about
single-threaded objects (stobjs) in ACL2 before proceeding;
see stobj.

The effect of this event is to introduce a new single-threaded
object (i.e., a ``stobj''), named name, and the associated
recognizers, creator, accessors, updaters, constants, and, for
fields of ARRAY type, length and resize functions.

The Single-Threaded Object Introduced

The defstobj event effectively introduces a new global
variable, named name, which has as its initial logical value
a list of k elements, where k is the number of ``field
descriptors'' provided. The elements are listed in the same order
in which the field descriptors appear. If the :type of a field
is (ARRAY type-spec (max)) then the corresponding element of
the stobj is initially a list of length max containing the value,
val, specified by :initially val. Otherwise, the :type
of the field is a type-spec and the corresponding element of
the stobj is the specified initial value val. (The actual
representation of the stobj in the underlying Lisp may be quite
different; see stobj-example-2. For the moment we
focus entirely on the logical aspects of the object.)

In addition, the defstobj event introduces functions for
recognizing and creating the stobj and for recognizing, accessing,
and updating its fields. For fields of ARRAY type, length and
resize functions are also introduced. Constants are introduced that
correspond to the accessor functions.

Restrictions on the Field Descriptions in Defstobj

Each field descriptor is of the form:

(fieldi :TYPE typei :INITIALLY vali)

Note that the type and initial value are given in ``keyword
argument'' format and may be given in either order. The typei
and vali ``arguments'' are not evaluated. If omitted, the type
defaults to t (unrestricted) and the initial value defaults to
nil.

Each typei must be either a type-spec or else a list of
the form (ARRAY type-spec (max)). The latter forms are said to
be ``array types.'' Examples of legal typei are:

(INTEGERP 0 31)
(SIGNED-BYTE 31)
(ARRAY (SIGNED-BYTE 31) (16))

The typei describes the objects which are expected to occupy
the given field. Those objects in fieldi should satisfy
typei. We are more precise below about what we mean by
``expected.'' We first present the restrictions on typei and
vali.

Non-Array Types

When typei is a type-spec it restricts the contents,
x, of fieldi according to the ``meaning'' formula given in
the table for type-spec. For example, the first typei
above restricts the field to be an integer between 0 and 31,
inclusive. The second restricts the field to be an integer between
-2^30 and (2^30)-1, inclusive.

The initial value, vali, of a field description may be any ACL2
object but must satisfy typei. Note that vali is not a
form to be evaluated but an object. A form that evaluates to
vali could be written 'vali, but defstobj does not
expect you to write the quote mark. For example, the field
description

(days-off :initially (saturday sunday))

describes a field named days-off whose initial value is the list
consisting of the two symbols SATURDAY and SUNDAY. In
particular, the initial value is NOT obtained by applying the
function saturday to the variable sunday! Had we written

(days-off :initially '(saturday sunday))

it would be equivalent to writing

(days-off :initially (quote (saturday sunday)))

which would initialize the field to a list of length two, whose first
element is the symbol quote and whose second element is a list
containing the symbols saturday and sunday.

Array Types

When typei is of the form (ARRAY type-spec (max)), the
field is supposed to be a list of items, initially of length max,
each of which satisfies the indicated type-spec. Max must be a
non-negative integer less than (2^28)-1. We discuss this limitation
below. Thus,

(ARRAY (SIGNED-BYTE 31) (16))

restricts the field to be a list of integers, initially of length
16, where each integer in the list is a (SIGNED-BYTE 31). We
sometimes call such a list an ``array'' (because it is represented
as an array in the underlying Common Lisp). The elements of an
array field are indexed by position, starting at 0. Thus, the
maximum legal index of an array field is max-1.

Note that the ARRAY type requires that the max be enclosed
in parentheses. This makes ACL2's notation consistent with the
Common Lisp convention of describing the (multi-)dimensionality of
arrays. But ACL2 currently supports only single dimensional arrays
in stobjs.

For array fields, the initial value vali must be an object satisfying
the type-spec of the ARRAY description. The initial value
of the field is a list of max repetitions of vali.

Array fields can be ``resized,'' that is, their lengths can be
changed, if :resizable t is supplied as shown in the example and
General Form above. The new length must satisfy the same
restriction as does max, as described above. Each array field in a
defstobj event gives rise to a length function, which gives the
length of the field, and a resize function, which modifies the
length of the field if :resizable t was supplied with the field when
the defstobj was introduced and otherwise causes an error.

Array resizing is relatively slow, so we recommend using it somewhat
sparingly.

name must be a new symbol, each fieldi must be a symbol,
each typei must be a type-spec or (ARRAY type-spec (max)),
and each vali must be an object satisfying typei.

Roughly speaking, for each fieldi, a defstobj introduces a
recognizer function, an accessor function, and an updater function.
The accessor function, for example, takes the stobj and returns the
indicated component; the updater takes a new component value and the
stobj and return a new stobj with the component replaced by the new
value. But that summary is inaccurate for array fields.

The accessor function for an array field does not take the stobj
and return the indicated component array, which is a list of length
max. Instead, it takes an additional index argument and
returns the indicated element of the array component. Similarly,
the updater function for an array field takes an index, a new
value, and the stobj, and returns a new stobj with the indicated
element replaced by the new value.

These functions -- the recognizer, accessor, and updater, and also
length and resize functions in the case of array fields -- have
``default names.'' The default names depend on the field name,
fieldi, and on whether the field is an array field or not. For
clarity, suppose fieldi is named c. The default names are
shown below in calls, which also indicate the arities of the
functions. In the expressions, we use x as the object to be
recognized by field recognizers, i as an array index, v as
the ``new value'' to be installed by an updater, and name as the
single-threaded object.

Finally, a recognizer and a creator for the entire single-threaded
object are introduced. The creator returns the initial stobj, but
may only be used in limited contexts; see with-local-stobj. If
the single-threaded object is named name, then the default names
and arities are as shown below.

introduces a stobj named $S. The stobj has two fields, X and
A. The A field is an array. The X field contains an
integer and is initially 0. The A field contains a list of
integers, each between 0 and 9, inclusively. Initially, each of the
three elements of the A field is 9.

If you do not like the default names listed above you may use the
optional :renaming alist to substitute names of your own
choosing. Each element of alist should be of the form
(fn1 fn2), where fn1 is a default name and fn2 is your choice
for that name.

Note that even though the renaming alist substitutes ``XACCESSOR''
for ``X'' the updater for the X field is still called
``UPDATE-X.'' That is because the renaming is applied to the
default function names, not to the field descriptors in the
event.

Use of the :renaming alist may be necessary to avoid name
clashes between the default names and and pre-existing function
symbols.

Constants

Defstobj events also introduce constant definitions
(see defconst). One constant is introduced for each accessor
function by prefixing and suffixing a `*' character on the function
name. The value of that constant is the position of the field being
accessed. For example, if the accessor functions are a, b, and c,
in that order, then the following constant definitions are introduced.

(defconst *a* 0)
(defconst *b* 1)
(defconst *c* 2)

These constants are used for certain calls of nth and update-nth
that are displayed to the user in proof output. For example, for
stobj st with accessor functions a, b, and c, in that order, the
term (nth '2 st) would be printed during a proof as (nth *c* st).
Also see term, in particular the discussion there of untranslated
terms, and see nth-aliases-table.

Inspecting the Effects of a Defstobj

Because the stobj functions are introduced as ``sub-events'' of the
defstobj the history commands :pe and :pc
will not print the definitions of these functions but will print
the superior defstobj event. To see the definitions of these
functions use the history command :pcb!.

To see an s-expression containing the definitions what constitute the raw
Lisp implementation of the event, evaluate the form

(nth 4 (global-val 'cltl-command (w state)))

immediately after the defstobj event has been processed.

A defstobj is considered redundant only if the name, field descriptors,
renaming alist, and inline flag are identical to a previously executed
defstobj. Note that a redundant defstobj does not reset the
stobj fields to their initial values.

Inlining and Performance

The :inline keyword argument controls whether or not accessor, updater,
and length functions are inlined (as macros under the hood, in raw Lisp). If
:inline t is provided then these are inlined; otherwise they are not.
The advantage of inlining is potentially better performance; there have been
contrived examples, doing essentially nothing except accessing and updating
array fields, where inlining reduced the time by a factor of 10 or more; and
inlining has sped up realistic examples by a factor of at least 2. Inlining
may get within a factor of 2 of C execution times for such contrived
examples, and within a few percent of C execution times on realistic
examples.

A drawback to inlining is that redefinition may not work as expected, much as
redefinition may not work as expected for macros: defined functions that call
a macro, or inlined stobj function, will not see a subsequent redefinition of
the macro or inlined function. Another drawback to inlining is that because
inlined functions are implemented as macros in raw Lisp, tracing
(see trace$) will not show their calls. These drawbacks are avoided by
default, but the user who is not concerned about them is advised to specify
:inline t.